1. Field of the Invention
The present invention relates to an electromagnetic switch and, more particularly, to an electromagnetic switch having a contact shaft with an integral plunger.
2. Description of the Prior Art
Referring to FIG. 5 a conventional electromagnetic switch 1 comprises a tubular outer case 2 with an integral wall 2a, and a plunger (moving core) 3 with an integral contact rod 5 received through a central opening formed in the wall 2a in the tubular outer case 2. As shown in FIG. 6, a hook 4 is received slidably in the central hole 3a of the plunger 3. A moving contact 6 is fixed to a contact holding member 7, which in turn is pressed by a spring 9 against a stopping member 8 attached to the extremity of the contact rod 5. The contact holding member 7 is supported slidably by a stationary core 10 fixedly fitted in the outer case 1. A coil bobbin 12 mounted with a solenoid 11 is fitted in the outer case 2 so as to receive the plunger 3 therein for axial sliding movement. A cap 13 is fixed to the open end of the outer case 2 and defines a contact chamber 14 together with the stationary core 10. A stationary contact 15 is attached to the cap 13 opposite to the moving contact 6. A rubber packing 16 is held between the cap 13 and the stationary core 10 to seal the contact chamber 14.
Referring to FIG. 7 showing the electromagnetic switch 1 as combined with a starter motor for a vehicle, a lever 17 has one end engaging the hook 4 of the plunger 3 and the other end engaging the rear end of an overrunning clutch 18 axially slidably mounted on the output shaft 21 of an electric motor 22. The clockwise swing of the lever 17 is limited by a stop ring 19. A driving pinion 20 is mounted fixedly on the inner race of the overrunning clutch 18.
When the starting switch, not shown, of the vehicle is closed, the solenoid coil 11 of the electromagnetic switch 1 is energized to produce a magnetic field, and the plunger 3 is attracted to the stationary core 10 by the magnetic field as shown in FIG. 7. The attraction of the plunger 3 by the stationary core 10 causes the lever 17 to swing counterclockwise to push the driving pinion 20 to the right, as viewed in FIG. 7. When the plunger 3 is attracted to the stationary core 10, the moving contact 6 comes into contact with the stationary contact 15 to connect the electric motor 22 through the contacts 6 and 15 to a power supply, not shown. When the starter switch is opened to de-energize the solenoid 11, the plunger 3 is released from the stationary core 10 and is returned to its initial position by a return spring.
Incidentally, when the plunger 3 is at the initial position as shown in FIG. 5, the difference .DELTA.l=l.sub.1 -l.sub.2 (wiping allowance), where l.sub.1 is the distance between the plunger 3 at the initial position and the stationary core 10, and l.sub.2 is the distance between the moving contact 6 and the stationary contact 15, generally is on the order of 1 mm. The wiping allowance .DELTA.l decreases gradually with time due to the abrasion of the moving contact 6 and the stationary contact 15, and when .DELTA.l&lt;0, faulty contact occurs. Therefore, the wiping allowance .DELTA.l must be determined taking into consideration the abrasion of the moving contact 6 and the stationary contact 15. The leverage k=a.sub.2 /a.sub.1, where a.sub.1 is the lever length of one of the arms of the lever engaging the hook 4, and a.sub.2 is the lever length of the other arm of the lever 17 engaging the rear end of the overrunning clutch 18, of the lever 17, a play B.sub.1 between the lever 17 and the hook 4, and a play B.sub.2 between the lever 17 and the stopper ring 19 must meet an inequality: B.sub.1 +B.sub.2 /k&gt;.DELTA.l, which is an essential condition for the electromagnetic switch 1. Accordingly, the electromagnetic switch 1 must be fabricated so that the wiping allowance .DELTA.l meets an inequality: B.sub.1 +B.sub.2 /k&gt;.DELTA.l&gt;0. However, the value of the wiping allowance .DELTA.l is dependent on the accuracies of a plurality of parts associated with the wiping allowance .DELTA.l, and, in some cases, the wiping allowance .DELTA.l deviates from the foregoing range owing to the cumulative variation in sizes of the associated parts within the respective tolerances. Accordingly, a plurality of plungers with an integral contact rod differing from each other in size l.sub.3 (FIG. 6) are prepared befor fabricating, and one of then is used selectively in fabricating the electromagnetic switch 1 to adjust the wiping allowance .DELTA.l to an appropriate value.
The conventional electromagnetic switch 1 thus constructed has the following disadvantages.
(1) A plurality of plungers with an integral contact rod having sizes in a range of l.sub.3 .+-..alpha. must always be kept in stock.
(2) Several electromagnetic switches produced in the initial stage of a lot production process, in some cases, are rejected as faulty in the wiping allowance .DELTA.l, because the wiping allowance .DELTA.l is measured in inspecting the electromagnetic switches in the final stage of fabrication after the completion of assembling the electromagnetic switches.